Corona discharge for electro-static charging

ABSTRACT

Apparatus for charging an insulating layer comprising an ion source for simultaneously producing a corona discharge of positive and negative ions and an acceleration electrode for extracting negative ions to uniformly charge the insulating layer. The ion source comprises at least one main electrode, an auxiliary electrode and a counter electrode coaxial to the main and auxiliary electrodes, and the accelerating electrode is arranged at the remote side of the layer to be charged. The apparatus further comprises deflecting electrodes and a plate electrode further acting upon the distribution of charge across the insulating layer.

United States Patent 1191 Hansen et a1.

1 Jan. 2, 1973 [54] CORONA DISCHARGE FOR ELECTRO- STATIC CHARGING [75]Inventors: Norbert Ernst Fritz Hansen, Roetgen; Dieter Wadow, Aachen,both of Germany [73] Assignee: U.S. Philips York,'N.Y.

22 Filed: Jan. 25, 1971 211 Appl.No.: 109,120

Corporation, New

[30] Foreign Application Priority Data.

Feb. 21, 1970 Netherlands ..7002477 [52] U.S. Cl... ..250/49.5 ZC,317/262 A s 1 Int. Cl. ..G03g 15/02 [58] Field of Search ..250/49.5 ZC,49.5 TC; 317/262 A [56] References Cited UNITED STATES PATENTS 7/1967Gundlach ..250/49.5 X

3,358,289 12/1967 Lee.. ..250/49.5 X 3,489,895 1/1970 3,163,753 12/19643,196,270 7/1965 Rosenthal ..250/49.5

Primary Examiner-William F. Lindqu ist Attorney-Frank R. Trifari [57]ABSTRACT Apparatus for charging an insulating layer comprising an ionsource for simultaneously producing a corona discharge of positive andnegative ions and an acceleration electrode for extracting negative ionsto uniformly charge the insulating layer. The ion source comprises atleast one main electrode, an auxiliary electrode and a counter electrodecoaxial to the main and auxiliary electrodes, and the acceleratingelectrode is arranged at the remote side of the layer to be charged. Theapparatus further comprises deflecting electrodes and a plate electrodefurther acting upon the distribution of charge across the insulatinglayer.

10 Claims, 3 Drawing Figures PA TENT ED 2 I975 3. 7 08 661 sum 1 or 2NOR BERT E.F. MKEQII DIETER WADOW BYWZ AGENT CORONA DISCHARGE FORELECTRO-STATIC CHARGING The invention relates to a device for chargingan insulating layer, particularly for electro-photography, said devicecomprising an ion source having .at least one main electrode forcoronadischarge and at least one counter-electrode and comprisingfurthermore a acceleration electrode on the side of the layer remotefrom the ion source. It is important for the charge applied by such adevice to the insulating layer to be uniformly distributed across saidlayer. In known devices it is endeavored to achieve this by moving theinsulating layer relatively to the corona electrodes, for example, byusing a layer in the form of a belt passing along a roller see U. S.Pat. No. 2,701,764 or by moving to and fro the suction electrode in theform of a flat plate supporting the layer beneath the electrodes asdescribed in British Pat. Specification 696,515. This British PatentSpecification also discloses measures for improving the reproduceabilityof the charging process with the aid of a grid between the electrodesand the insulating layer. In this embodiment the layer can be chargedfairly uniformly up to a comparatively high potential, for example, afew hundred volts. However, with some types of layers better results areobtained when the layer is charged only up to a potential of a few tensof volts. With these charging devices this is only possible at theexpense of uniformity. Moreover, it is sometimes undesirable for thelayer to be moved during the charging process, for example, if it isdesired to charge and expose simultaneously.

An important cause of the non-uniform charge distribution resides inthat the corona discharge is strongly localized so that the ions formedmigrate from a few separate areas towards the layer. This gives rise tothe formation of a few ion channels between the electrodes and thelayer, which are filled with space charges having the same polaritiesand hence repelling one another; separate charge region therefore beingformed on the layer. The object of the invention is to avoid theformation of these channels, so that a continuous charge distribution isproduced on a stationary plate, which distribution can then be furthersmoothed by additional measures. According to the invention this isachieved by providing means for producing simultaneously positive andnegative ions.

The invention will be described more fully with reference to thedrawing, in which:

FIG. 1 is a sectional view of a charging device embodying the invention,

FIG. 2 is an enlarged plan view of the ion source of FIG. 1 and FIG. 3illustrates the distribution of the ion flow across the insulatinglayer.

The charging device shown in FIG. 1 comprises an ion source 1 comprisingneedle-shaped main electrodes 3 see also FIG. 2 a cylindricalcounter-electrode 5 and a needle-shaped auxiliary electrode 7, mountedin an insulating block 9. In-the embodiment shown the main electrodes 3,as is clearly shown in FIG. 2, are uniformly distributed along thesheath of a cylinder, which is coaxial to the counter-electrode 5, whilethe auxiliary electrode 7 coincides with the axes of the two cylinders.As will be apparent from FIG. 1, the dimensions of the ion source 1 arepreferably considerably smaller than its distance from a accelerationelectrode 11, which is provided on its side facing the source with aninsulating layer not shown for the sake of clarity Also by this largedistance the operation of the ion source 1 is not affected by thepotential of the acceleration electrode 11. The electrodes are fed fromhighvoltage sources not shown for the sake of clarity which are allconnected on one side to a common earth-connection point 13, to which isalso connected the counter-electrode 5. The main electrodes 3 receivefrom point 15 a negative voltage of, for example, 10 kV and theauxiliary electrode 7 receives a positive voltage of, for example, 8.5kV from point 17. The value of these voltages is chosen so that a coronadischarge occurs at all needle-shaped electrodes, so that negative ionsemerge from the main electrodes 3 and positive ions from the auxiliaryelectrode 7. It is thus avoided that relatively repelling space chargeconcentrations of identical ions are produced; a homogeneous ion cloudis formed which contains both positive and negative ions. Since theacceleration electrode 11 receives a positive voltage of, for example,13 kV from point 19, it is capable of extracting negative ions from thecloud so that the insulating layer is charged negatively. This charge iscontinuously distributed across the layer and it is not, as in saidknown devices, concentrated in a number of discrete charge regions. Inthe arrangement shown in FIG. 1, comprising one ion source which issmall as compared with the transverse dimension of the accelerationelectrode, a greater charge is produced at the center of the layer thanat the sides. This might be improved by using more ion sources, but asimpler and cheaper solution consists in the provision of a'plurality ofannular deflection electrodes 21, which receive from point 23 apreferably variable positive voltage of, for example, 10.5 kV so thatthey cause the ion beam to spread in fan-shaped fashion on its path fromthe source to the layer. This permits further smoothing of the alreadycontinuous charge distribution on the insulator to obtain a more uniformdistribution. If desired, a further improvement may be obtained byarranging a plate electrode 25 on the side of the plate-shapedacceleration electrode 11 remote from the ion source, said electrode 25also receiving a preferably variable positive voltage of, for example, 11 kV from point 27.

In order to obtain full effect of these control-possibilities it shouldbe possible to measure the ion flow to different parts of the insulatinglayer. For this purpose probes 29 are provided, which are connected topoint 19 through measuring instruments 31 for very low currents, forexample, electrometers.

FIG. 3 is a graph made by means of said probes 29, in which the probecurrent is plotted as a function of the distance from the center of acircular suction electrode having a diameter of 36.5 cms. The ion sourcewas at a distance of 28 cms from the suction electrode and had adiameter of 12 cms. It will be apparent that practically throughout thelayer the ion flow and hence the charge substantially does not change.

By varying the charging time it is now possible to charge uniformly thelayer to any desired potential up wards of a few volts.

What is claimed is:

1. Apparatus for charging an insulating layer comprising an ion sourcefor simultaneously producing a homogeneous ion cloud of positive andnegative ions adjacent to but spaced from one side of said insulatinglayer, said ion source comprising at least two electrodes for producinga corona discharge of negative and positive ions, respectively, meansfor supplying potentials to said electrodes, an accelerating electrodeon the side of said insulating layer remote from said ion source, andmeans to supply a potential to said accelerating electrode at which onlynegative ions are extracted from said ion cloud thereby uniformlydistributing negative charges on said insulating layer.

2. Apparatus as claimed in claim 1 wherein said two electrodes forproducing a corona discharge of negative and positive ions comprises atleast one main electrode for producing a corona discharge of negativeions and an auxiliary electrode for producing a corona discharge ofpositive ions within said corona discharge of negative ions.

3. Apparatus as claimed in claim 1 wherein said ion source is of givendimensions and further comprises a counter electrode for forming saidcorona discharge into an ion cloud, the distance between said ion sourceand said acceleration electrode being substantially greater than thedimensions of said ion source so that the potential of said accelerationelectrode has no effect on said ion source, the potential supplied tosaid accelerating electrode being positive relative to said counterelectrode.

4. Apparatus as claimed in claim 3 wherein said two electrodes forproducing a corona discharge of negative and positive ions comprises atleast one main electrode for producing a corona discharge of negativeions and an auxiliary electrode for producing a corona discharge ofpositive ions within said corona discharge of negative ions, said mainelectrode and said auxiliary electrode comprisingneedle-shaped,.parallel conductors and said counter-electrode beingshaped as a tube surrounding said conductors.

5. Apparatus as claimed in claim 4 wherein said counter-electrode has acylindrical shape, said auxiliary electrode lying along the longitudinalaxis of. said cylinder and said main electrodes being evenly distributedalong a cylindrical surface coaxial to said counter-electrode.

6. Apparatus as claimed in claim 5 wherein said main electrode islocated at the center of a straight line between the auxiliary electrodeand thecounter-electrode.

7. Apparatus as claimed in claim 1 further comprising at least onedeflecting electrode positioned between said ion source and saidinsulating layer outside of the paths of the ions from said ion sourcefor influencing the distribution of charge across said layer.

8. Apparatus as claimed in claim 7 wherein said deflecting electrodesare parallel to each other and are spaced apart from the insulatinglayer by different distances, the further the deflection electrodes arespaced from the insulating layer, the greater the distances between thedeflection electrodes and the connecting line between the ion sourceandthe insulating layer.

9. Apparatus as claimed in claim 1 further comprising a flatplateelectrode positioned on the side of the acceleration electroderemote from the ion source, said plate electrode having lateraldimensions substantially larger than those of said acceleratingelectrode and extending parallel to said acceleration electrode formfluencing the charge distribution across the insulating layer.

10. Apparatus as claimed in claim 1 further comprising probes coupled tosaid acceleration electrode to measure very low currents indicative ofthe ion HOW to different parts of said insulating layer.

1. Apparatus for charging an insulating layer comprising an ion sourcefor simultaneously producing a homogeneous ion cloud of positive andnegative ions adjacent to but spaced from one side of said insulatinglayer, said ion source comprising at least two electrodes for producinga corona discharge of negative and positive ions, respectively, meansfor supplying potentials to said electrodes, an accelerating electrodeon the side of said insulating layer remote from said ion source, andmeans to supply a potential to said accelerating electrode at which onlynegative ions are extracted from said ion cloud thereby uniformlydistributing negative charges on said insulating layer.
 2. Apparatus asclaimed in claim 1 wherein said two electrodes for producing a coronadischarge of negative and positive ions comprises at least one mainelectrode for producing a corona discharge of negative ions and anauxiliary electrode for producing a corona discharge of positive ionswithin said corona discharge of negative ions.
 3. Apparatus as claimedin claim 1 wherein said ion source is of given dimensions and furthercomprises a counter electrode for forming said corona discharge into anion cloud, the distance between said ion source and said accelerationelectrode being substantially greater than the dimensions of said ionsource so that the potential of said acceleration electrode has noeffect on said ion source, the potential supplied to said acceleratingelectrode being positive relative to said counter electrode. 4.Apparatus as claimed in claim 3 wherein said two electrodes forproducing a corona discharge of negative and positive ions comprises atleast one main electrode for producing a corona discharge of negativeions and an auxiliary electrode for producing a corona discharge ofpositive ions within said corona discharge of negative ions, said mainelectrode and said auxiliary electrode comprising needle-shaped,parallel conductors and said counter-electrode being shaped as a tubesurrounding said conductors.
 5. Apparatus as claimed in claim 4 whereinsaid counter-electrode has a cylindrical shape, said auxiliary electrodelying along the longitudinal axis of said cylinder and said mainelectrodes being evenly distributed along a cylindrical surface coaxialto said counter-electrode.
 6. Apparatus as claimed in claim 5 whereinsaid main electrode is located at the center of a straight line betweenthe auxiliary electrode and the counter-electrode.
 7. Apparatus asclaimed in claim 1 further comprising at least one deflecting electrodepositioned between said ion source and said insulating layer outside ofthe paths of the ions from said ion source for influencing thedistribution of charge across said layer.
 8. Apparatus as claimed inclaim 7 wherein said deflecting electrodes are parallel to each otherand are spaced apart from the insulating layer by different distances,the further the deflection electrodes are spaced from the insulatinglayer, the greater the distances between the deflection electrodes andthe connecting line between the ion source and the insulating layer. 9.Apparatus as claimed in claim 1 further comprising a flat plateelectrode positioned on the side of the acceleration electrode remotefrom the ion source, said plate electrode having lateral dimensionssubstantially larger than those of said accelerating electrode andextending parallel to said acceleration electrode for influencing thecharge distribution across the insulating layer.
 10. Apparatus asclaimed in claim 1 further comprising probes coupled to saidacceleration electrode to measure very low currents indicative of theion flow to different parts of said insulating layer.